Methods for making an electroactive device fabricated with a nanotube film electrode

ABSTRACT

Disclosed is a single wall carbon nanotube (SWCNT) film electrode (FE), all-organic electroactive device systems fabricated with the SWNT-FE, and methods for making same. The SWCNT can be replaced by other types of nanotubes. The SWCNT film can be obtained by filtering SWCNT solution onto the surface of an anodized alumina membrane. A freestanding flexible SWCNT film can be collected by breaking up this brittle membrane. The conductivity of this SWCNT film can advantageously be higher than 280 S/cm. An electroactive polymer (EAP) actuator layered with the SWNT-FE shows a higher electric field-induced strain than an EAP layered with metal electrodes because the flexible SWNT-FE relieves the restraint of the displacement of the polymeric active layer as compared to the metal electrode. In addition, if thin enough, the SWNT-FE is transparent in the visible light range, thus making it suitable for use in actuators used in optical devices.

CROSS REFERENCE TO RELATED APPLICATION

This patent application is a divisional of copending U.S. patentapplication Ser. No. 13/938,622, filed Jul. 10, 2013, which claims thebenefit of U.S. patent application Ser. No. 13/284,061, filed Oct. 28,2011; which claims the benefit of U.S. patent application Ser. No.11/937,155, filed Nov. 8, 2007, which claims the benefit of U.S.Provisional Application Nos. 60/857,531, filed Nov. 8, 2006, and60/984,027 filed Oct. 31, 2007; the contents of each of the foregoingapplications are hereby incorporated by reference in their entireties.

ORIGIN OF THE INVENTION

The invention described herein was made by an employee of the UnitedStates Government and may be manufactured and used by or for theGovernment of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

FIELD OF THE INVENTION

This invention relates generally to electroactive polymeric devices andcompliant electrodes for these devices. More specifically, the inventionrelates to electroactive polymeric devices utilizing highly compliantnanotube film electrodes and methods for making same.

DESCRIPTION OF THE RELATED ART

As an interest in high performance polymeric electroactive devicesincreases, a request for new electrode materials has emerged. Knownelectroactive polymeric devices typically use metal electrodes, such assilver and gold, to provide electric fields. These metal electrodesoften inhibit the displacement (elongation or contraction) of theirelectroactive layer because of less compliance (greater stiffness(modulus)) of the metal electrodes than the active polymer itself. Thus,the actual electric field-induced strain output of these devices withmetal electrodes is always smaller than what they could intrinsicallyprovide.

Conducting polymers have been used as alternative electrodes forelectroactive polymeric devices. The conducting polymers relieved therestraint of movement in the polymeric active layer because theircompliance is similar to that of the active polymeric layer, andexhibited higher strain than metal electrodes did. However, theseconducting polymers have a disadvantage of low conductivity at hightemperatures because of dehydration phenomena and dedoping, andtherefore are unable to be used for applications which require highthermal stability. Therefore, a need existed for an alternativeelectrode with less stiffness than the conventional metallic electrodesand with good thermal stability.

SUMMARY OF THE INVENTION

In accordance with at least one embodiment of the present invention anovel freestanding flexible single-walled carbon nanotubes (SWCNT) filmelectrode (SWCNT-FE) is provided. This inventive electrode shows highconductivity and good thermal stability with comparable compliance topolymeric active layers. Additionally, in accordance with at least oneembodiment of the present invention, a novel high performanceall-organic electroactive device (or system) is provided, fabricatedwith the SWCNT-FE. Methods for the preparation of the electrode anddevice are also provided within the scope of the present invention.Features and advantages of the inventions will be apparent from thefollowing detailed description taken in conjunction with the followingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram depicting the preparation of anall-organic electroactive device system in accordance with at least oneembodiment of the present invention;

FIG. 1B shows a photograph of a prototype of a transparent electroactivedevice fabricated with an EAP active layer and SWNT film electrodes, inaccordance with at least one embodiment of the present invention;

FIG. 2A shows a cross-sectional SEM image of SWCNT-FE after pressing at600 psi, in accordance with at least one embodiment of the presentinvention;

FIG. 2B shows a more detailed image of the pulled and porous networkedSWCNTs shown in FIG. 2A;

FIG. 2C shows a cross sectional SEM image of SWCNT-FE after pressing at6000 psi, in accordance with at least one embodiment of the presentinvention;

FIG. 3A is a graph depicting the dielectric constant of an inventive EAPlayered with SWCNT-FE as a function of temperature and frequency;

FIG. 3B is a graph depicting the dielectric constant of an EAP layeredwith metal electrodes as a function of temperature and frequency;

FIG. 4 is a graph depicting the electric field-induced strain of an EAPlayered with metal electrodes, and with an inventive SWCNT-FE;

FIGS. 5A and 5B are photographs of a freestanding flexible SWCNT-FE inaccordance with at least one embodiment of the present invention, afterit is removed from the membrane (shown in 5B), in accordance with thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

Shown in the drawings and described herein in detail are advantageousembodiments of the present invention. It should be understood that thepresent invention is susceptible of embodiments in many different formsand thus the present disclosure is to be considered as anexemplification of the principles of the invention and is not intendedto limit the broad aspect of the invention to the embodiments describedand illustrated herein.

Referring now to the drawings, FIG. 1A is a diagram depicting thepreparation of an all-organic electroactive device system in accordancewith at least one embodiment of the present invention (such as thedevice 10 shown in FIG. 1B). A SWCNT film can be prepared by a methodsimilar to the method set forth in A. G. Rinzler and Z. Chen, U.S.Patent Application Publication 20040197546 (Oct. 7, 2004), the entirecontents of which are hereby incorporated by reference. However, inaccordance with the present invention, unlike U.S. ApplicationPublication 20040197546, no surfactant is required to develop the SWNTfilm and no solvent is necessary to isolate the SWNT film from thefilter membrane (by dissolving the membrane). Additionally, it should beunderstood that while the described inventive embodiment utilizesSWCNTs, it is nevertheless within the scope of the present invention toreplace the SWCNTs with multi-walled carbon nanotubes (MWCNT) or fewwall carbon nanotubes (FWCNT). Additionally, other types of conductivenanotubes can be used in the instant invention, for example, boronnanotubes, boron carbon nitride nanotubes, and/or boron nitridenanotubes.

To explain, in accordance with one inventive method, the inventiveelectrode can be developed as follows. First, SWCNTs can be dispersed inan solvent, such as N,N-Dimethylacetamide (DMAc), under sonication andfiltered onto the surface of a brittle or breakable porous membrane,such as an anodized alumina membrane (pore size: 0.2 μm), to form a SWNTfilm on the membrane. Advantageous dispersion methods (not requiringsurfactants or covalent bonds), and choices of appropriate solvents,which can be utilized in accordance with the present invention, can befound in co-pending U.S. patent applications, namely, application Ser.No. 10/288,797, entitled “Electrically Conductive, Optically transparentPolymer/Carbon Nanotube Composites and Process for Preparation Thereof,”filed Nov. 1, 2002; application Ser. No. 11/432,201, entitled“Dispersions of Carbon Nanotubes in Polymer Matrices,” filed on May 11,2006; and application Ser. No. 11/644,019, entitled “Nanocomposites fromStable Dispersions of Carbon Nanotubes in Polymeric Matrices UsingDispersion Interaction,” filed on Dec. 22, 2006. These three pendingU.S. patent applications are incorporated herein by reference as setforth in their entirety.

After the formation of the SWCNT film on the membrane (for example,through the removal of the solvent in a known manner), a freestandingSWCNT film can then be easily delaminated by breaking the brittle (e.g.alumina) membrane. This breaking can be accomplished in a manner thatwould be known to one skilled in the art, the result of which is shownin FIG. 5B. In one advantageous embodiment, the delaminated SWCNT filmwill have the conductivity of about 280 S/cm. The thickness of the SWCNTfilm can be controlled from several tens of nanometers to severalhundreds of micrometers by adjusting the concentration and quantity ofSWCNT solution used. Adjusting the concentration and quantity of SWCNTsolution used will also affect the final conductivity of the SWCNT film.Additionally, adjusting the thickness of the film will affect thetransparency of the film. For example, it was found that a 2 μm thickSWCNT film was opaque (black), while a 300 nm thick SWCNT film was foundto be optically transparent.

In accordance with at least one advantageous embodiment of the presentinvention, as shown in FIG. 1A, an inventive all-organic electroactivedevice (SWCNT-FE/EAP/SWCNT-FE) can be fabricated with an electroactivepolymer (EAP) active layer 11 and the SWCNT films 12, 13 by pressing,for example, at 600, 3000 or 6000 psi, as shown in FIG. 1A. Inaccordance with one embodiment of the invention, the pressingtemperature and time were 230° C. and 2 min., respectively. All of thesample specimens were preheated at 230° C. for 20 minutes prior topressing. Silicone elastomer plates 14, 15 (e.g., 3 mm thick) can beused on the press plate surfaces for better contact adhesion between theSWCNT film and the actuating layer. This polymeric electroactive devicelayered with the SWCNT-FE can serve as an actuator. However, it shouldbe understood that it is within the scope of the present invention thatother devices (such as sensors, transducers, etc.) could also befabricated utilizing the novel methods and inventions set forth herein.Additionally, the embodiment shown in FIG. 1A is merely illustrative ofone possible device design. As is known in the art, depending upon thedesired application and geometry, the device could be configured in manydifferent ways, for example, with different numbers, sizes, shapes andlocations of active layers and electrodes (e.g., round, interdigitated,etc.). Also, different types of active layers could be utilized,depending upon the application for which the particular device isdesigned. Examples of various active layers can be found in U.S. Pat.Nos. 5,891,581 and 5,909,905, as well as pending U.S. patent applicationSer. No. 11/076,460, entitled “Sensing/Actuating Materials Made fromCarbon Nanotube Polymer Composites and Methods for Making Same,” filedMar. 3, 2005, and pending U.S. patent application Ser. No. 11/081,888,entitled, “Multilayer Electroactive Polymer Composite Material,” filedon Mar. 9, 2005. These patents and applications are hereby incorporatedby reference as if set forth in their entirety herein

FIGS. 2A and 2C show SEM images of cross-sections of inventive SWCNT-FEs22, 23 after pressing at 600 psi and at 6000 psi, respectively. FIG. 2Bshows a more detailed image of the pulled and porous networked SWCNTsshown in FIG. 2A. The cross-section of the SWCNT-FE 23 pressed under6000 psi (FIG. 2C) was denser than that pressed under 600 psi (FIG. 2A).

The density (modulus or compliance) of the SWNT-FE can be controlled byadjusting the fabrication pressure. As explained more fully below, it isanticipated that less dense (higher compliance) SWCNT-FE can presentless constraint to the displacement by more closely matching the modulusof the polymeric active layers. Therefore, in at least one advantageousembodiment of the present invention, the fabrication pressure isadjusted to produce a SWCNT-FE with a compliance (and modulus)substantially matching the compliance of the device's active layer. Inthis manner a device can be fabricated with substantially uniformcompliance throughout, thereby potentially improving the performance ofthe device, for example, by maximizing the electric field-induced strainoutput of the device.

Most conducting polymers become unstable above 120° C., and lose theirconductivity significantly. However, for many applications, the actuatorsystem must be able to function at temperatures even up to 200° C. orhigher. Therefore, it was necessary to examine if SWCNT-FE functions ata broad range of temperatures and frequencies. The performance of theSWCNT film as an electrode was evaluated by measuring the dielectricproperties of an Electroactive Polymer (EAP) layered with the SWNT filmas an electrode (SWCNT-FE) at a broad range of temperatures (from 25° C.to 280° C.) and frequencies (from 1 KHz to 1 MHz). The temperature andfrequency dependence of the dielectric constant for an EAP layered withSWCNT-FE is shown in FIG. 3A, which is almost the same as that of thedielectric properties of the same EAP layered with gold electrodes (FIG.3B). The dielectric constant remained constant up to 220° C., and thenincreased. The increase of the dielectric constant at 220° C. is due tothe glass transition temperature (T_(g)) of the EAP ((β-CN)APB/ODPApolyimide, U.S. Pat. No. 5,891,581 Joycelyn O. Simpson and Terry St.Clair, “Thermally stable, piezoelectric and pyroelectric polymericsubstrates”). Above T_(g), dipoles have a higher mobility and show ahigher dielectric constant. Additionally, as frequency decreases, it isbelieved that these dipoles have enough time to orient themselves underan applied electric field, creating a higher dielectric constant.Thermally stable dielectric properties suggest that SWCNT-FE is suitablefor high temperature applications at least up to 220° C. SWCNT usuallydo not oxidize below a temperature of about 400° C., therefore, if ahigher stability polymer was used a fabricated device could potentiallyfunction at a much higher temperature. Success in the use of knownconducting polymer electrodes at high temperatures (above 100° C.) hasrarely been reported. Conducting polymers have a disadvantage of lowconductivity at high temperatures because of dehydration phenomena anddedoping, and therefore are unable to be used for applications whichrequire high thermal stability.

Electric field-induced strain values for EAP layered with metalelectrodes and SWCNT-FE are shown in FIG. 4. The EAP actuator layeredwith the SWCNT-FE showed a higher electric field-induced strain than anEAP layered with metal electrodes under identical measurement conditionssince the flexible, highly compliant SWCNT-FE relieves the restraint ofthe displacement of the polymeric active layer compared to the metalelectrode. In addition, as explained above, when prepared thin enough,the SWCNT-FE can be transparent in the visible light range (see FIG.1B). Actuators fabricated with the transparent SWCNT-FE can be used inoptical devices such as optical switches and modulators.

As shown in FIG. 4, the out-of-plane strain (S₃₃) through the filmthickness was plotted as a function of applied electric field strength.The strain (S₃₃) of EAP layered with silver electrodes increasedquadratically with increasing applied electric field, indicating thatthe strain is mainly electrostrictively originated. The electrostrictivecoefficient (M₃₃) vs. EAP layered with silver electrodes, calculatedfrom a slope in a plot of strain (S₃₃) vs. the square of electric fieldstrength (E), S₃₃=M₃₃E², was 1.58E-15 m²/V². The strain of EAP layeredwith SWCNT-FE after pressing at 600 psi increased more rapidly than thatlayered with silver electrodes. The electrostrictive coefficient (M₃₃)of this SWCNT-FE system (600 psi) was 3.86E-15 m²/V², more than 2 timeshigher than those of EAP layered with silver electrodes. Thissignificant increase in strain indicates that less dense SWCNT-FE seemedto restrain the displacement of the active layer less. Additionally, asthe pressure of the fabrication of the EAP/SWCNT-FE system increased,the strain decreased, since the SWNT-FE became denser and couldconstrain the displacement of the active layer more (FIG. 2A-2C). At6000 psi, the strain value was close to that of EAP with the silverelectrodes, which indicates that the modulus of the SWCNT-FE prepared at6000 psi was close to that of silver electrodes at the interface.

Additionally, all-organic electroactive device systems fabricated withsingle wall carbon nanotube (SWCNT) films used as electrodes have shownenhanced electroactive performance in comparison with conventionalelectroactive device system fabricated with metal electrodes. SWCNT canbe replaced by multi wall carbon nanotubes (MWCNT) or few wall carbonnanotubes (FWCNT). Further, SWCNT film electrodes (SWCNT-FE) have shownreliable capability as an electrode in an electrical device at hightemperatures suitable for aerospace applications. Additionally, othertypes of conductive nanotubes may also be used in these applications,such as boron nanotubes, boron carbon nitride nanotubes, and/orboron-nitride nanotubes.

As explained above, certain mechanical properties of SWCNT-FE (e.g.Young's modulus) can be controlled by adjusting the magnitude of thefabrication pressure, to form resultant electrodes with mechanicalproperties substantially matching with those of employed active layers.Additionally, in accordance with at least one embodiment of theinvention, higher mechanical properties (e.g. Young's modulus, strength,elongation at break, durability, robustness, etc.) of SWCNT-FE can beachieved by using acid-treated SWNTs (which are commercially available)and post-sintering at above 350° C. temperature. A freestanding flexibleSWCNT-FE with high conductivity has been developed. One such inventivefreestanding flexible SWCNT-FE 52 is shown in FIGS. 5A and 5B, afterdelamination by breaking the brittle membrane 53. FIG. 5B shows thefreestanding flexible SWCNT-FE 52 sitting on the broken membrane 53. Asexplained above, a freestanding SWCNT-FE can be pressed during thefabrication of a device, or, in the alternative, it could beindependently pressed in order to achieve a desired thickness,conductivity, compliance, transparency, etc

As explained above, the thickness of the SWCNT film is easily controlledby the concentration and quantity of SWCNT solution, and it can rangefrom about several tens of nanometers to about several hundreds ofmicrometers. The SWCNT film which was thinner than several hundreds ofnanometer was found to be transparent. Therefore, the freestandingflexible transparent SWCNT film electrodes (SWCNT-FE) enables theinventive all-organic electroactive devices to be used in opticaldevices such as optical switches and modulators.

Potential applications for an all-organic electroactive devicefabricated with carbon nanotubes, e.g., single wall carbon nanotube(SWCNT) film electrodes (SWCNT-FE), include electromechanical energyconversion devices such as electromechanical sensors and actuators,transducers, sonars, medical devices, prosthetics, artificial muscles,and materials for vibration and noise control. The high performanceinventive all-organic electroactive devices possess many advantages overpiezoceramic and shape-memory alloys owing to their light weight,conformability, high toughness, and tailorable properties needed inthese applications. In addition, the transparency of the novelall-organic electroactive devices fabricated with SWNT-FE enables themto be used in optical devices such as optical switches and modulators.The freestanding flexible SWCNT-FE can provide a great degree of freedomto fabricate a variety of complex electroactive devices.

Although only a few exemplary embodiments of this invention have beendescribed in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. Additionally, it should be understood that the useof the term “invention” herein should not be limited to the singular,but rather, where applicable, it is meant to include the plural“inventions” as well. Further, in the claims, means-plus-function andstep-plus-function clauses are intended to cover the structures or actsdescribed herein as performing the recited function and not onlystructural equivalents, but also equivalent structures.

What is claimed as new and desired to be secured by Letters Patent ofthe United States is:
 1. A method for making an electroactive devicehaving a nanotube film electrode, comprising the steps of: providing atleast one nanotube film electrode; providing at least one active layer;placing said at least one nanotube film electrode in contact with saidat least one active layer; applying sufficient pressure to said at leastone nanotube film electrode and said at least one active layer so as toproduce an electroactive device having a substantially uniformcompliance throughout, wherein said step of applying sufficient pressurecomprises controlling a density of said at least one nanotube filmelectrode by said sufficient pressure to control a compliance of said atleast one nanotube film electrode.
 2. The method of claim 1, comprisingthe step of heating said at least one nanotube film electrode and saidat least one active layer.
 3. The method of claim 1, wherein saidsufficient pressure ranges between about 600 to about 6000 psi.
 4. Themethod of claim 1, wherein said step of applying sufficient pressurecomprises utilizing silicone elastomer plates on press plates.
 5. Themethod of claim 1, where said active layer comprises an electroactivepolymer.
 6. The method of claim 1, wherein the nanotube film electrodeconsists essentially of at least one or more of: single-walled carbonnanotubes; multi-walled carbon nanotubes; few walled carbon nanotubes;boron nanotubes; boron carbon nitride nanotubes, and boron nitridenanotubes.
 7. The method of claim 1, wherein the nanotube film electrodeis prepared by a process comprising the steps of: dispersing conductivenanotubes in a surfactant-free solvent under sonication to form asolution; providing a breakable porous membrane; filtering the solutiononto the membrane; and delaminating the nanotube film electrode from themembrane by physically breaking the membrane.